When an aircraft is moving on the ground, in order to reach the takeoff runway or attain its parking position, it follows traffic lanes (designated by the term taxiway) comprising ground markings standardized by the international organizations and notably a yellow center line. By procedure, the movement of the aircraft is performed under the command of the ground control which assigns the route to each of the aircraft. The crew is in charge of applying the manoeuvre by sight, that is to say by relying on the view at its disposal through the apertures of the flight deck. At night or in degraded meteorological conditions, the crew relies on the luminous aids available on the airport to follow the assigned path, monitor the approach to intersections or runways and to center the aircraft on the taxiway used.
Under particularly unfavourable conditions, known by the OACI terminology as level 4 visibility conditions, the pilot is not in a position to ensure the guidance of his aircraft. In such cases, the current procedure consists in assisting each aircraft with a pilot vehicle which travels in front of it, with a perfect knowledge of the airport and of the route to be followed. The pilot of the aircraft is then reduced to following the luminous signalling means of the pilot vehicle. Having regard to the number of available vehicles and the inability of the aircraft to steer in an autonomous manner, these operations are carried out at highly reduced speeds of movement and with a reduced number of assisted aircraft.
An approach to improving the safety conditions of aircraft movements under such conditions, while maintaining a higher traffic throughput, consists in assisting the pilot to follow the path assigned to him by ground control. Accordingly, it is necessary to provide him with dynamic parameters of the aircraft, relating to the ground markings, such as the position and the orientation of the aircraft or the rolling speed. Several systems, described hereinafter, already make it possible to measure these dynamic parameters but their ground accuracy is not sufficient to aid the pilot or to envisage automatic guidance of the aircraft on the traffic lanes.
Systems of ADIRS type (the initials being the acronym of the expression Air Data Inertial Reference System) are measurement equipment based on pressure measurements performed on various parts of an aircraft, typically at the front and on the sides. The measured pressure differences make it possible to deduce flight parameters such as the speed of the aircraft. These apparatuses are accurate in the flight phase, but in the ground rolling phase, at low velocity, their accuracy degrades appreciably.
Satellite positioning systems such as GPS (Global Positioning System) offer an accuracy of the order of 10 metres, which is insufficient during the rolling phases. This type of system is tied to the visibility of the constellation of satellites and, in the aeronautical context, the receivers have been optimized to guarantee maximum performance during flight, with accuracies of the order of a hundred metres, but with very high integrity. During the rolling phases, the necessary accuracy is sub-metric for the following of a path and one of the particular features of the rolling phases arises from the fact that the locating system also relies on the satellites which are seen on very low orbits above the horizon which have a tendency to degrade the accuracy of the measurement.
Guidance systems based on feedback with respect to database elements are subject to potential database errors and to the accuracy of the aircraft position measurements. The currently available airport databases have neither the accuracy nor integrity necessary for guidance operations. Additionally, the accuracy positioning arrangement requires the use of differential systems. An airport reference point position measurement, performed by a GPS for example, is transmitted to the aircraft. The measurement of the deviation between the aircraft and the reference point greatly reduces the measurement errors. Nevertheless, this technique makes it necessary to equip the aircraft and the airports and does not render the navigation of the aircraft autonomous.
The invention is aimed at alleviating the locating problems cited above by proposing a device and a method, based on the acquisition of images, and making it possible to ascertain the relative position and the orientation of the aircraft with respect to ground markings, that are therefore independent of all databases or means of global location. The use of the device according to the invention affords high accuracy in location and makes it possible to envisage automatic guidance of the aircraft on traffic lanes. The method according to the invention calculates deviations between the aircraft and the center line of the taxiway that it is following. These deviations allow the pilot to guide the aircraft on the ground, including in difficult meteorological conditions. The invention makes it possible to provide the input data necessary to a ground guidance system by relying on a measurement carried out on the real world and which is independent of any system of navigation, location or bases of position charts of airport elements.